This invention relates to pre-pit detection units fit for use in recording-playback apparatus intended for high-density recording media such as DVD-R and DVD-RW and more particularly to a pre-pit detection unit ensuring that pre-pits can be detected even though the amplitude level of a radial push-pull signal fluctuates.
As disclosed in the Examined Japanese Patent Application No. Hei 10-283638, for example, a recordingxe2x80x94playback apparatus intended for high-density recording media such as DVD-R and DVD-RW has a self-contained pre-pit detection unit ensuring that pre-pits on a recording medium can be detected even though beam intensity is in the state of either mark or space period.
FIGS. 11 to 13 show a specific example of such a pre-pit detection unit as mentioned above; and FIG. 14, an operating waveform chart. As shown in FIG. 11, the basic arrangement of the pre-pit detection unit is such that a pre-pit detection circuit 300 corresponding to the beam intensity in the mark period and a pre-pit detection circuit 400 corresponding to the beam intensity during the space period are provided in parallel on the output side of a push-pull circuit 200 for generating a pre-data signal and that the OR of the outputs of both pre-pit detection circuits 300 and 400 is finally outputted as a pre-pit detection signal (l).
In FIG. 11, reference numeral 101 denotes a quadrified detector (light receiving element) for outputting a four-system light receiving signal comprising A to D areas; 102, an adder (e.g. an adder using an OP amplifier) for generating a light receiving signal (a-1) in the left-hand area with respect to the push-pull circuit 200 by adding the A-area light receiving signal and the D-area light receiving signal of the quadrified detector 101; 103, an adder for generating a light receiving signal (a-2) in the right-hand area with respect to the push-pull circuit 200 by adding the B-area light receiving signal and the C-area light receiving signal of the quadrified detector 101; 104, a DVD encoder for encoding recording data by utilizing digital modulation technology; 105, a gate signal generator 105 for generating gate signal (h-1 and h-2) according to NRZI and NRZICK signals obtainable from the DVD encoder 104; and 106, an OR operator 106 for obtaining the OR of the outputs (i-1 and i-2) of both the pre-pit detection circuits 300 and 400.
FIG. 12 shows the internal structure of the push-pull circuit 200. As shown in FIG. 12, the push-pull circuit 200 includes a substractor (e.g., an adder using an OP amplifier) 201 for generating a radial push-pull signal (d) as a pre-data signal by subtracting the light receiving signal (a-1) in the left-hand area and the light receiving signal (a-2) in the right-hand area.
FIG. 13 shows the internal structure of the pre-pit detection circuits 300 and 400. As shown in FIG. 13, the pre-pit detection circuit 300 corresponding to the beam intensity in the mark period includes a (fixed gain) amplifier 301 for amplifying the radial push-pull signal (d) outputted from the push-pull circuit 200 by means of a constant gain, a comparator 303 for detecting pre-pits by comparing the output signal (f-1) of the amplifier 301 with a mark equivalent voltage (VMARK) as a threshold voltage (VT1), and a sampling gate 304 that is opened and closed under the control of the gate signal (h-1) so as to pass the output signal (g-1) of the comparator 303 therethrough. Similarly, the pre-pit detection circuit 400 corresponding to the beam intensity in the space period includes a (fixed gain) amplifier 401 for amplifying the radial push-pull signal (d) outputted from the push-pull circuit 200 by means of a constant gain, a comparator 403 for detecting pre-pits by comparing the output signal (f-2) of the amplifier 401 with a mark equivalent voltage (VSPACE) as a threshold voltage (VT2), and a sampling gate 404 that is opened and closed under the control of the gate signal (h-2) so as to pass the output signal (g-2) of the comparator 403 therethrough.
In the above case, a so-called floating binarization system is employed for the comparing operation in the comparators 303 and 403. Consequently, the mark equivalent voltage (VMARK) and the space equivalent voltage (VSPACE) are generated by shifting the level of the radial push-pull signal (d) by a low component.
In such a recording-playback apparatus as mentioned above, the balance of the quantity of light that is incident on the light receiving element and reflected from the disk collapses because of variations in the environment of use that cause optical axis fluctuations to a laser oscillator and because the presence of optical aberration in the pickup as well as the presence of internal-external aberration in the reflectance of the disk. In consequence, the amplitude level of the radial push-pull signal (d) may fluctuate (may cause disturbance).
When the situation above arises in the prior art pre-pit detection unit, the output signals (f-1 and f-2) of the respective amplifiers 301 and 401 are directly susceptible to the influence of fluctuations in the amplitude level of the radial push-pull signal (d). On the other hand, values of the mark equivalent voltage (VMARK) and space equivalent voltage (VSPACE) as the threshold voltages VT1 and VT2 of the comparators 303 and 403 disposed at the following stage of the respective amplifiers 301 and 401 are generated by level-shifting the low component of the radial push-pull signal (d) by a predetermined value, the fluctuations of the radial push-pull signal (d) with respect to the maximum amplitude lever will have to be dull.
When the amplitude level of the radial push-pull signal (d) sharply fluctuates as shown in the waveform chart of FIG. 14, the output signal (f-1) of the amplifier 301 consequently follows the fluctuation and fluctuates, whereupon an abnormal pulse whose pulse width and phase are not normal or a pre-pit pulse that should not be present (error detection) will appear in a pre-pit detection signal (l) resulting from gating of the output signal (g-1) of the comparator 303 with the sampling gate 304; the problem is that address retrieval and spindle control may be impeded by the mistaken recognition of pre-format data.
An object of the present invention with special attention directed to the foregoing problems in the prior art pre-pit detection unit is to provide a pre-pit detection unit capable of detecting a pre-pit accurately even though the amplitude level of a radial push-pull signal fluctuates.
In order to accomplish the object above, a pre-pit detection unit according to the invention comprises a push-pull circuit for generating a radial push-pull signal including a pre-data signal according to each of lateral outputs of a light receiving element, a first pre-pit detection circuit for beam intensity in a mark period for detecting a pre-pit corresponding to the beam intensity in the mark period according to the radial push-pull signal obtained from the push-pull circuit, and a second pre-pit detection circuit for beam intensity in a space period for detecting a pre-pit corresponding to the beam intensity in the space period according to the radial push-pull signal obtained from the push-pull circuit, so that the OR of outputs of both pre-pit detection circuits is outputted as a pre-pit detection signal, and is characterized in that a first and a second AGC circuit for making the maximum amplitude of the radial push-pull signal obtainable from the push-pull circuit coincide with a reference value are provided in the preceding stages of pre-pit detecting comparators in the first and the second pre-pit detection circuits respectively.
The pre-pit detection circuit for the beam intensity in the mark period and the pre-pit detection circuit for the beam intensity in the space period may be provided in parallel on the output side of the push-pull circuit for generating the pre-data signal.
With this arrangement, since the comparison input level of the pre-pit detecting comparator is maintained within the reference value even though the amplitude level of the radial push-pull signal fluctuates, an abnormal pulse whose pulse width and phase are not normal or a pre-pit pulse that should not be present (error detection) is prevented from appearing in the pre-pit detection signal, which results in preventing address retrieval and spindle control from being impeded by the mistaken recognition of pre-format data as long as the threshold value as a comparison standard for the pre-pit detecting comparator is properly set.
The threshold value of the pre-pit detecting comparator is preferably set according to a reference value that the maximum amplitude of the push-pull signal should coincide with.
With this arrangement, it is therefore possible to keep constant the maximum amplitude level of the radial push-pull signal and also to improve pre-pit detection accuracy accordingly as a constant level difference is always maintained between the maximum amplitude of the push-pull signal and the comparing threshold value.
The AGC circuits for making the maximum amplitude of the signal obtained from the light receiving element coincide with the reference value is preferably provided in each input system of a differential operator forming the push-pull circuit for generating the pre-data signal.
With this arrangement, it is therefore possible to restrain any abnormal radial push-pull signal from being outputted from the push-pull circuit itself as much as possible, whereupon pre-pit detection accuracy is also improvable further.
The first and the second AGC circuits include a variable gain amplifier for adjusting the amplitude of the radial push-pull signal, an amplitude detector for detecting the amplitude of the output signal of the variable gain amplifier, and a differential operator circuit for generating a gain control signal for controlling the gain of the variable gain amplifier according to the output value of the amplitude detector and a predetermined reference value, and the amplitude detector preferably includes a first peak hold circuit which is positioned at the preceding stage and has a small time constant, and a second peak hold circuit which is positioned at the following stage and has a large time constant.
This arrangement ensures that a sharp variable component equivalent to the pre-pit is caught and properly held over pre-pit appearance intervals.
An amplitude limiting circuit is preferably placed between the first and second peak hold circuits.
With this arrangement, the possibility of mistaking any defect on the disk for a pre-pit becomes reducible.
Further, in order to accomplish the objective above, a pre-pit detection method comprises the steps of: generating a radial push-pull signal including a pre-data signal according to each of lateral outputs of a light receiving element by a push-pull circuit, detecting a pre-pit corresponding to beam intensity in a mark period according to the radial push-pull signal obtained from the push-pull circuit by a first pre-pit detection circuit for the beam intensity in the mark period for, and detecting a pre-pit corresponding to a beam intensity in a space period according to the radial push-pull signal obtained from the push-pull circuit by a second pre-pit detection circuit for the beam intensity in the space period, so that the OR of outputs of both pre-pit detection circuits is outputted as a pre-pit detection signal, and making the maximum amplitude of the radial push-pull signal obtainable from the push-pull circuit coincide with a reference value by a first and a second AGC circuits provided in the preceding stages of pre-pit detecting comparators in the first and the second pre-pit detection circuits respectively.
The pre-pit detection circuit for the beam intensity in the mark period and the pre-pit detection circuit for the beam intensity in the space period may be provided in parallel on the output side of the push-pull circuit for generating the pre-data signal.
With this method, since the comparison input level of the pre-pit detecting comparator is maintained within the reference value even though the amplitude level of the radial push-pull signal fluctuates, an abnormal pulse whose pulse width and phase are not normal or a pre-pit pulse that should not be present (error detection) is prevented from appearing in the pre-pit detection signal, which results in preventing address retrieval and spindle control from being impeded by the mistaken recognition of preformat data as long as the threshold value as a comparison standard for the pre-pit detecting comparator is properly set.
The threshold value of the pre-pit detecting comparator is preferably set according to a reference value that the maximum amplitude of the push-pull signal should coincide with.
With this method, it is therefore possible to keep constant the maximum amplitude level of the radial push-pull signal and also to improve pre-pit detection accuracy accordingly as a constant level difference is always maintained between the maximum amplitude of the push-pull signal and the comparing threshold value.
The AGC circuits for making the maximum amplitude of the signal obtained from the light receiving element coincide with the reference value is preferably provided in each input system of a differential operator forming the push-pull circuit for generating the pre-data signal.
With this method, it is therefore possible to restrain any abnormal radial push-pull signal from being outputted from the push-pull circuit itself as much as possible, whereupon pre-pit detection accuracy is also improvable further.
In the first and the second AGC circuits; adjusting the amplitude of the radial push-pull signal by a variable gain amplifier, detecting the amplitude of the output signal of the variable gain amplifier by an amplitude detector, and generating a gain control signal for controlling the gain of the variable gain amplifier according to the output value of the amplitude detector and a predetermined reference value by a differential operator circuit, wherein the amplitude detector includes; a first peak hold circuit which is positioned at the preceding stage and has a small time constant, and a second peak hold circuit which is positioned at the following stage and has a large time constant.
This method ensures that a sharp variable component equivalent to the pre-pit is caught and properly held over pre-pit appearance intervals.
An amplitude limiting circuit is preferably placed between the first and second peak hold circuits.
With this method, the possibility of mistaking any defect on the disk for a pre-pit becomes reducible.